1. Field of the Invention
This invention relates to a hydrofoil blade for use in a paper making machine of the type wherein hydrofoil blades are positioned beneath a forming medium and extended in the cross machine direction relative to the forming medium for draining water through the forming medium from a paper web being formed on the forming medium and for forming the paper web.
2. Description of the Prior Art
In the typical Foundrinier papermaking machine, an aqueous suspension of fibers, called the "stock" is flowed from a headbox onto a traveling Fourdrinier wire or medium, generally a woven belt of wire and/or synthetic material, to form a continuous sheet of paper or paper-like material. In this connection, the expression "paper or paper-like material" is used in a broad or generic sense and is intended to include such items as paper, kraft, board, pulp sheets and non-woven sheet-like structures. As the stock travels along the Fourdrinier wire, formation of a paper web occurs, as much of the water content of the stock is removed by draining. Water removal is enhanced by the use of such well-known devices as hydrofoil blades, table rolls and/or suction devices. This invention relates to hydrofoil blades.
The hydrofoil blades used in papermaking perform two functions. The first function is to create a vacuum pulse over the downward inclined face of the hydrofoil blade. This pulse removes a portion of the white water from the lower side of the stock which lays upon the forming medium and causes fibers to be laid down and formed into a web. The amount of such water removal and web formation over a given hydrofoil blade is small, and therefore a considerable number of blades is required to form all of the fibers in the stock suspension into a two dimensional web. For example, the use of ten to fifty hydrofoil blades is not uncommon. In other words, the sheet forming process is a step-by-step filtration process as the forming medium travels over the hydrofoil blades, with some of the fibers in the lower portion of the stock suspension over the partially-formed web being added to the web at each successive foil blade. The average net change in fiber concentration or consistency of this process ranges from the headbox consistency, which is usually about 0.4 percent to about 1 percent, up to about 2.5 percent.
The second function of a hydrofoil blade is to maintain the fibers which are still is suspension throughout the forming process in an as-well-as dispersed condition as possible; i.e., in a deflocculated condition. This function is extremely important as fibers in the 0.5-2.0 percent consistency range have a strong tendency to flocculate into clumps on their own in a matter of milliseconds once the fiber dispersive forces have decayed. This flocculation causes the final paper to be highly non-uniform or flocculated in appearance.
The realization in the 1970's that papermaking stocks at commercially used consistencies reflocculate in milliseconds once floc dispersing forces on the papermaking machine decay has led to an array of devices to deliver such forces into the stock remaining to be formed into a web throughout the sheet forming process. The two key requirements of these floc dispersing forces are (1) that their size or scale is sufficiently small so that they only break up the fiber flocs, but do not disrupt the overall large scale mass of the suspension, and (2) that their intensity is sized likewise.
Both the intensity and scale of the turbulence generated by conventional foil blades of the type first described by Wrist, US Pat. No. 2,948,465 are a function of the square of both their angle to the forming fabric and the speed of the papermaking machine. As a result, the turbulence they generate is rarely optimum on papermachines producing a variety of grades over a wide speed range.
A further disadvantage of such conventional foils is that their dewatering rate and the intensity of the turbulence they generate are directly related to each other. That is, if more turbulence is required and a large foil angle is employed, then more dewatering is invariably obtained as well. Such an effect is often undesirable, especially during the early stages of sheet formation where considerable redispersion of the stock prior to sheet formation is often highly desirable. This is usually the case, for example, with older, overloaded headboxes delivering suspensions which are poorly dispersed and contain large scale eddy currents.
One device developed recently in an effort to overcome these shortcomings of such conventional foils is the multi-step foil blade described in Kallmes, US Pat. No. 4,687,549. Such foils dewater stock in a controllable manner without generating any turbulence whatsoever. Its use in a redispersing system relies on the continuous cross machine direction shear generated by the phase-changing ridges produced either by a serrated slice or a formation shower to keep the stock dispersed throughout the sheet-forming process. This cross machine direction shear acts on the stock remaining to be formed into a sheet in a manner similar to the well-known shake of slow running papermachines.
One of the key characteristics of a sheet forming process employing a serrated slice or a formation shower to keep the stock dispersed and the multi-step foil described in US Pat. No. 4,687,549 to provide turbulence-free controlled dewatering only is that it separates these two functions. That is, the pressure of the formation shower controls the intensity of the cross machine direction shear generated while the overall angle of inclination of the multi-step foil blade to the forming fabric controls the rate of dewatering.
The cross machine direction shear generated by the phase changes of the ridges produced by either a serrated slice or a formation shower are highly effective in improving the formation quality of virtually all types of paper. However, both serrated slices and formation showers have certain undesirable characteristics. Serrated slices are fixed structures which cannot be adjusted at will, and their design, like that of foil blades, is not optimum at all machine speeds on multi-grade papermachines. Formation showers also have their limitations in that, for example, their nozzles often plug, and they tend to catch stock sprayed off the forming fabric which can build up fiber clumps on them and then drop off to cause sheet breaks. Thus, there are many papermakers who shy away from using these devices for practical operating reasons.
It is desirable to overcome the foregoing shortcomings by providing a multi-step foil blade which produces floc-dispersing turbulence of controllable scale and intensity, and simultaneously independently controls the rate of dewatering.